Add a juju restore subcommand.

This is the CLI portion for the work we are doing on cleaning up the juju-restore plugin (and turning it into a normal command).  For now it just stubs out the actual call to the restore client.
While we do write our function documentation starting with the name of the function, I don't think "Restore restores" reads correctly when someone runs "juju help restore".

If you look at "juju help status" it reads:
"This command will report on the runtime state of various system entities."
...

juju help set-env says:
Updates the environment of a running Juju instance.  Multiple key/value pairs...

I don't know that we've been super consistent and have great help text. But something more like:

Restore a backup that was previously created with "juju backup".
This command creates a new state server and arranges for it to replace the previous state server for an environment.
It also ensures all known instances are configured to treat the new state server as their master.
...